Add basic HAL types

Add schema vars for the basic HAL types of Resource, Link, and
Curie. See the [HAL Specification][hal] for full details

[hal]: http://tools.ietf.org/html/draft-kelly-json-hal-06
Use the markdown codox plugin to support links in doc strings
Merge pull request #4 from cddr/hal-schema

Add schema vars for HAL entities